GSN Regular Membership Meeting—May 17

 

“The location is again at Great Basin’s Taps & Tanks, 1155 S. Rock Blvd. Reno, NV. Drinks @ 6:00 pm, APPETIZERS @ 6:30 pm, Talk @ 7:15 pm. Speaker, Mike Ressel, NBMG/UNR. Title: “Carlin-type Deposits As Part of the Great Basin’s Eocene Metallogeny“. DRINKS SPONSORED BY: FALCON DRILLING AND HARRIS EXPLORATION DRILLING! Appetizers only for $15—prepay online or pay at the door. GSN students are free. For more info contact Laura Ruud at gsn@gsnv.org or 775-323-3500. Details on page 3. To PREPAY for appetizers please click on this link: https://squareup.com/store/GSNV

No reservations required but avoid the line by pre-paying!”

Speaker: Mike Ressel, NBMG

Topic: Carlin-type Deposits As Part of the Great Basin’s Eocene Metallogeny

Mike Ressel, Curtis Johnson, Elizabeth Hollingsworth, Christopher Henry, and Philipp Ruprecht, Nevada Bureau of Mines and Geology and/or University of Nevada, Reno

Abstract: Late Eocene (~42-34 Ma) ore deposits have contributed a majority of the Great Basin’s precious- and base-metal production through 2015, including about 6.2 Kt Au (199 Moz), 27.6 Kt Ag (887 Moz) , 17.5 Mt Cu, and 0.5 Mt Mo. The production of these four metals alone from Eocene deposits would have a present-day value of over $400 billion (USD), ~64% of which is attributable to gold, 29% to copper. Eocene ore deposits are restricted to the northern half of the Great Basin, coincident with arc magmatism and the earliest stages of Cenozoic crustal extension. Curiously, metal production from Eocene deposits is strikingly polarized, with 79% of Au production derived from mines in north-central Nevada, and 98% of Cu and 86% of Ag derived from mines in north-central Utah. Most Nevada Au production is from three major Au belts that host giant sedimentary rock-hosted Carlin-type Au deposits: Carlin, Battle Mountain-Eureka, and Getchell. Three major Utah districts produced the bulk of Cu, Mo, Au, and Ag (Oquirrh, Park City, and Tintic), with the vast majority of Cu, Mo, and Au coming from the Bingham Canyon porphyry system. In addition, Eocene deposits in northern Nevada contain more than 1.68 Kt of Au (54 Moz) and 0.67 Mt Mo in reserves, and Bingham Canyon contains reserves of about 2.72 Mt of Cu and 0.15 Mt of Mo.

Perhaps more than any other deposit type in the Great Basin, Carlin-type gold deposits are enigmatic. After more than five decades of mining more than 150 Moz of gold from Carlin-type deposits in north-central Nevada, we continue to debate what one is, resorting to terms such as Carlin-like to qualify our uncertainty. As a result, the rest of the exploration world is even more baffled by them. Uncertainty has hindered exploration beyond the Great Basin because of lack of a well-constrained exploration model that incorporates not only elements of existing descriptive models but also the syn-mineral regional geologic framework of type deposits in Nevada.

We assess findings contributing toward a global Carlin-type deposit exploration model. Key regional components of the model based on Great Basin geology are: 1) age and age progression of Carlin-type gold mineralization, 2) spatial and temporal association of Carlin-type deposits with other gold-rich deposits, and 3) the regional geologic setting of gold deposition including crustal architecture and syn-mineral tectonism and magmatism. The progressive change in Eocene ore deposits from Au- to Cu-dominant across the central to eastern parts of the northern Great Basin is extraordinary and has major exploration implications. This change in metals parallels changes in the geochemical and isotopic character of magmatism across the Eocene arc, although rocks are grossly similar in terms of their mineralogy and bulk composition. We preliminarily interpret west to east variations in Eocene igneous chemistry and styles of metallization as interrelated, with contrasting chemistries reflecting major differences in architecture and composition of the crust through which magmas traveled.

Thus, the range of Eocene sedimentary rock-hosted, disseminated gold deposits (SHDGs), including Carlin-type deposits, in northern Nevada defines a distinct intrusion-related gold metallogeny that contrasts with the “classic” deposits formed in many other continental arc settings, including the eastern Great Basin in Utah, which are typified by porphyry Cu and Cu-Mo, polymetallic skarns and replacements, and high- and intermediate-sulfidation epithermal Au-Ag deposits. The reduced mineralogy and geochemistry of ores and Au-dominant or Au-only character of SHDGs in northern Nevada infer overall reduced ore fluids that fundamentally differ from highly oxidized fluids indicated for porphyry-related systems. We suggest that emplacement of the Eocene arc far inboard of the plate margin and into kilometers-thick carbonaceous slope and basinal rocks of the Neoproterozoic through Paleozoic passive margin progressively modified and chemically reduced mid-crustal magmas from mafic to silicic compositions through assimilation of reduced crust.

New Geologic Map: Lookout Mountain, Ratto Ridge, and Rocky Canyon, Eureka County

Open-File Report 14-8
Preliminary geologic and alteration maps of Lookout Mountain, Ratto Ridge and Rocky Canyon, southern Eureka mining district, Eureka County, Nevada

by Russell V. Di Fiori, Sean P. Long, John L. Muntean, and Gary P. Edmondo, 2014

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Identification of favorable structural settings for Carlin-type gold deposits is fundamental for future exploration.  In this study, mapping and structural analysis were performed in the southern part of the Eureka mining district in east-central Nevada, in order to understand geometric and temporal relationships between structural systems and Carlin-type mineralization.  Geologic and alteration maps at a scale of 1:10,000 of a ~3.5 km (east-west) by ~8 km (north-south) region were generated, along with cross sections that illustrate pre- and post-extensional deformation geometry.  This project bridges a gap between recent 1:24,000-scale mapping and <1:500-scale mapping performed in an active exploration campaign.

The stratigraphy of the map area consists of ~4 km of Cambrian-Devonian carbonate and siliciclastic rocks, which are unconformably overlain and intruded by late Eocene silicic volcanic rocks.  Four structural systems are identified, consisting of Early Cretaceous contractional structures and three separate sets of normal faults: 1) 1st-order, kilometer-scale offset, down-to-the-west normal faults, including the Lookout Mountain and Dugout Tunnel faults, 2) 2nd-order, 10’s to 100’s meter-scale offset, north-striking normal faults, including the Rocky Canyon, Oswego, and East Ratto Ridge fault systems, and 3) a set of 3rd-order, meter-scale offset, east-striking normal faults that cut jasperoid bodies of presumed late Eocene age.  The 1st- and 2nd-order faults are interpreted to be contemporary, cut Late Cretaceous intrusions and an associated contact metamorphic aureole, and are overlapped by a late Eocene, subvolcanic unconformity.

In addition to lithology and structure, specific types of hydrothermal alteration and mineralization were mapped, including silicification, decarbonatization, dolomitization, quartz/calcite-veining, argillization, and the introduction of sulfides and their limonite weathering products.  Carlin-type replacement mineralization, primarily hosted within Cambrian carbonate rocks, occurs in a series of deposits in the southern part of the map area.  The deposits are associated with decarbonatization, silicification and jasperoid formation, and argillization, and are constrained to late Eocene or older by the overlap and intrusion of dated volcanic rocks.

The map area contains a km-scale, faulted relay-ramp of 2nd-order faults that transfer slip between synthetic 1st-order faults. Within accommodation zones, wall-damage zones are predicted to provide hydrothermal fluid pathways and therefore localize mineralization.  The footwall of the Lookout Mountain fault, which contains the majority of Carlin-type deposits identified in the map area, contains a set of antithetic, 2nd-order normal faults, which is interpreted as a wall-damage zone that was responsible for controlling fluid flow that led to mineralization.

The southern Eureka mining district contains several favorable structural conditions for Carlin-type gold mineralization, including: 1) normal fault systems that predate or are contemporary with late Eocene gold mineralization, and 2) complex normal fault interactions in an accommodation zone, including zones of dense fault intersections, antithetic normal faults, and fault-damage zones.  These structural conditions were fundamental for generating a network of open-system fluid pathways, which created an ideal structural architecture for Carlin-type mineralization, and can be used as predictive tools for exploration elsewhere.

This project was supported by Timberline Resources Corporation and the Geological Society of America.

Open-File Report 14-8, two plates, scale 1:10,000; plate one: 56 x 41 inches, plate two: 29 x 41 inches; folded or rolled, $35.00

Available free on the Web or purchase here: http://www.nbmg.unr.edu/sales/pbsdtls.php?sku=OF14-%208

Nevada Mineral Industry 2012

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MI-2012: The Nevada Mineral Industry 2012, by John L. Muntean, David A. Davis, Lisa Shevenell, and Benjamin McDonald, 2014

Starting in 1979, NBMG has issued annual reports that describe mineral and geothermal activities and accomplishments in Nevada, and include statistics of known gold and silver deposits.

This report describes mineral, oil and gas, and geothermal activities and accomplishments in Nevada in 2012: production statistics, exploration and development including drilling for petroleum and geothermal resources, discoveries of orebodies, new mines opened, and expansion and other activities of existing mines. Statistics of known gold and silver deposits, and directories of mines and mills are included.

Available free on the Web: http://www.nbmg.unr.edu/dox/mi/12.pdf

Special Publication MI-2012, 177 pages (includes 19 pages in color)

New Director of Center for Research in Economic Geology (CREG)—John Muntean

Announcement from MSESE Director, Russ Fields (12-14-12): “I am pleased to announce that Dr. John Muntean has been named Director of the Ralph Roberts Center for Research in Economic Geology (CREG) at the Mackay School of Earth Sciences and Engine…

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Announcement from MSESE Director, Russ Fields (12-14-12):

“I am pleased to announce that Dr. John Muntean has been named Director of the Ralph Roberts Center for Research in Economic Geology (CREG) at the Mackay School of Earth Sciences and Engineering.  The Mackay School is part of the College of Science, University of Nevada, Reno.

Dr. Muntean will lead CREG in its mission to direct research in the field of economic geology specifically on the deposition of gold deposits. CREG is a unit of Mackay’s Department of Geological Sciences and Engineering.

Following is information about Dr. Muntean:

John Muntean is a Research Economic Geologist and Associate Professor with the Nevada Bureau of Mines and Geology at the University of Nevada, Reno.  He received his BS from Purdue University, his MS from the University of Michigan, and his PhD from Stanford University.  Before joining UNR in 2005, John worked 12 years in the mining industry for companies exploring for gold in Nevada, including Santa Fe Pacific, Homestake, and Placer Dome.  At Placer Dome, he provided technical support for Placer’s worldwide near-mine and long-term generative exploration efforts.  His research has been on epithermal, porphyry, and Carlin-type gold deposits in Nevada and Latin America.  At NBMG he has also been involved in mineral assessments, mapping projects, database compilations, and reporting on mineral exploration activities in Nevada.  He is an active member of the Society of Economic Geologists and the Geological Society of Nevada.

Please join me in welcoming John as the CREG director.  John’s email address is munteanj@unr.edu

I also wish to recognize Dr. Tommy Thompson for his leadership of CREG for approximately the last 16 years.  Dr. Thompson and his graduate students have worked on many of the questions on gold deposition and have advanced our collective understanding.  We wish him well as he continues to serve on the faculty of the Department of Geological Sciences and Engineering at Mackay.”

John Muntean would like to add:

“I look forward to working with any who are interested in helping CREG continue to develop and grow into a world-class research unit in the formation and exploration for ore deposits. I have many ideas that I’d like to bounce off people.  If I don’t contact you first, please contact me with your suggestions.  My email is munteanj@unr.edu. Thank you for your continuing support of the CREG program.”